Greater capacity has become very important in communication networks in recent years with the increasingly widespread use of the Internet and cellular telephones. One way to achieve greater capacity is to raise the transmission speed. A high-speed optical signal of 160 Gbps has been reported at the research stage.
The increase in transmission speeds dictates higher speed in the measurement devices used to measure transmission signals. Sampling oscilloscopes, which are one way to monitor signal waveforms, incorporate switching elements that are used to sample the signal under test. The current generation of sampling oscilloscopes uses varactor diodes as switching elements. Higher sampling rates are required to be able to monitor higher-frequency signal waveforms, but increasing the switching speed of varactor diodes is difficult.
Another known switching element is the photoconductive switch in which switching is performed by irradiating a semiconductor photoconductive switch element with short pulses of light generated by a high-speed laser. In one example, the semiconductor switching element was composed of GaAs grown at a low temperature of about 200xc2x0 C.
The optimum wavelength of light for irradiating a semiconductor switching element composed of low-temperature GaAs is about 850 nm. Short pulses of light of this wavelength can be generated by a Ti-sapphire laser or a mode-locked fiber laser whose light output is doubled in frequency by a separate second harmonic generation (SHG) element. Such lasers are capable of generating light with a pulse width as short as about 0.1 ps. However, a Ti-sapphire laser has a number of problems that make it unsuitable for practical use: it is bulky, it requires cooling water and its power output is unstable. A mode-locked fiber laser with a separate SHG element is costly and bulky because of its use of the SHG element.
A mode-locked fiber laser can be fabricated to generate light having a wavelength of about 1.55 xcexcm. Such a laser is compact, lightweight, and needs no cooling water. Also, the light generated by a mode-locked fiber laser has a highly-stable pulse width, low amplitude noise and low jitter. This light can also have an ultra-short pulse width and a high repetition rate.
However, it is impractical to use the light generated by a mode-locked fiber laser that generates light having a wavelength of 1.55 xcexcm to control a low-temperature grown GaAs semiconductor switching element because the low-temperature grown GaAs has a low absorptivity at such a long wavelength. Absorption attributable to defects in low-temperature GaAs, and two-photon absorption have been reported for light having a wavelength of 1.55 xcexcm, but the efficiency of these absorption mechanisms is too low for them to be used in a practical photoconductive switch.
What is needed, therefore, is a photoconductive switch and a semiconductor photoconductive switch element that have a compact, relatively simple construction and that can be controlled by light having a wavelength of about 1.55 xcexcm.
The invention provides a photoconductive switch element that comprises a photoconductive layer and a wavelength conversion element. The wavelength conversion element converts incident light having a first wavelength into activating light having a second wavelength at which the photoconductive layer has a greater absorptivity than at the first wavelength. The wavelength conversion element is integral with the photoconductive layer, or is in contact with the photoconductive layer, or is both integral with and in contact with the photoconductive layer.
The wavelength conversion element may include a nonlinear optical material that generates the activating light at the second harmonic of the incident light.
The photoconductive layer and the wavelength conversion element may include a compound semiconductor material, and the compound semiconductor material of at least the wavelength conversion element may have a (100) crystal axis that is tilted by at least 5 degrees relative to the direction of the incident light.
The photoconductive switch element may additionally comprise a substrate of single-crystal (n11) semiconductor material, where n is an integer.
At least the wavelength conversion element may include a layer of the compound semiconductor material grown on the substrate.
The photoconductive switch element may additionally comprise a substrate and at least the photoconductive layer may include an ion-implanted layer in the substrate.
The nonlinear optical material of the wavelength conversion element may be configured as a layer stacked on the photoconductive layer. The nonlinear optical material may be quasi-phase matched.
The nonlinear optical material may be quasi-phase matched, may be sized larger than the photoconductive layer, and may support the photoconductive layer in a location adjacent the end of the nonlinear optical material remote from the end at which the incident light is received.
The wavelength conversion element may include a first major surface via which the incident light is received and a second major surface opposite the first major surface, and the photoconductive layer may be bonded to the second major surface of the wavelength conversion element. The photoconductive layer may include a first major surface bonded to the wavelength conversion element and a second major surface opposite the first major surface, and the photoconductive switch element may additionally comprise electrodes located on the second major surface of the photoconductive layer.
The invention also provides a photoconductive switch that comprises a laser that generates light having a first wavelength and a photoconductive switch element arranged to receive the light generated by the laser as incident light. The photoconductive switch element includes a photoconductive layer and a wavelength conversion element. The photoconductive layer has a low absorptivity at the first wavelength. The wavelength conversion element converts the incident light into activating light having a second wavelength at which the photoconductive layer has a greater absorptivity than at the first wavelength. The wavelength conversion element is integral with the photoconductive layer, or in contact with the photoconductive layer, or both integral with and in contact with the photoconductive layer.
Low-cost lasers capable of generating ultra-short pulses of light generate such light in a wavelength range in which the semiconductor material of the photoconductive layer of a fast photoconductive switch has a low absorptivity. The photoconductive switch and photoconductive switch element according to the invention include a wavelength conversion element integral with the photoconductive layer or in contact with the photoconductive layer. The wavelength conversion element converts the wavelength of the incident light generated by the laser to one at which the semiconductor material of the photoconductive layer has a greater absorptivity than at the wavelength of the incident light. Thus, the invention provides a photoconductive switch and photoconductive switch element controllable by a low-cost laser capable of generating ultra-short pulses of light.